Although there are a large number of agents that inhibit microbial infections in culture or even when administered systemically, many of these agents are not effective when administered via topical ocular application as a result of inadequate penetration of the drug through the cornea. Even within a structural class the observed in vivo activity of a compound depends on both the anti-microbial activity and the ability of the compound to localize at the appropriate concentration in the affected tissue. The present invention is directed to the discovery of physical properties that control the topical ocular activity of drugs (e.g., fluoroquinolone antibiotics), as well as to the provision of a method for identifying compounds that have sufficient corneal penetration capability to be administered via topical application to the eye.
The single cellular epithelial layer of the cornea is the primary barrier to the trans-corneal penetration of drug molecules. Although a number of methods have been used to enhance penetration across the corneal epithelium, these methods tend to disrupt the intercellular connections that serve as a defensive barrier protecting the eye from invasions by pathogens. Disruptions in this barrier often result in toxicity which is amplified upon repeated application.
A superior approach is to enhance trans-celluar penetration by designing or identifying compounds with optimum physical properties. Compounds in solution or suspension need to partition from the topical formulation into the lipid rich cellular membrane of the corneal epithelium, traverse the cell and exit through the basolateral epithelial cell membrane. As long as the formulation is in contact with the exterior surface of the eye, the steep concentration gradient serves as the driving force for penetration into the cornea. In this limited time, the physical properties of the molecule in the formulation govern the rate of drug penetration into and through the corneal epithelium.
Aqueous solubility and lipophilicity are two factors that govern the rate a drug penetrates the cornea (FIG. 1). However, other physical properties of the drug molecule (e.g., pKa and distribution coefficient) may also have an impact on corneal permeability.
For most topical ocular drugs, the rate-limiting barrier to corneal penetration is the two top cell layers of the corneal epithelium which are lipoidal in nature (FIG. 1B). A drug's lipophilicity is estimated by its octanol/water partition coefficient, koc, though the logarithm of this value, log P, is more often reported. If one takes into consideration the pH of the aqueous phase, the proportion of drug in its non-ionized or preferentially absorbed form can be determined, along with the distribution coefficient, or DC (Table 1). Due to the tri-laminate structure of the corneal membrane which effectively blocks passive diffusion by most molecules, the optimal n-octanol/water log P range for transcellular corneal drug penetration is 2-3.16 
The pioneering work of Schoenwald found that corneal permeability is a function of lipophilicity for steroids17 and β-blockers.18 However, Wu et al.19 found no such correlation for a small set of cephalosporins. Both, Fukada et al.1 and Liu et al.20 demonstrated that corneal permeability correlates with fluoroquinolone lipophilicity. Also Ruiz-Garcia et al.21 observed that lipophilicity is the main factor governing intestinal fluoroquinolone absorption.
Several studies examining the tear concentrations and corneal penetration properties of fluoroquinolone antibiotics have been published.8-11 However, only a limited amount of information has been published regarding the corneal penetration properties of the new fourth-generation fluoroquinolones, moxifloxacin and gatifloxacin. The present inventors conducted a study to (i) investigate the physical properties underlying the superior corneal penetration of moxifloxacin and (ii) develop a method for predicting the corneal permeability of moxifloxacin and other fluoroquinolones using in vitro data and mathematical models. This work resulted in the development of a new and more reliable method for predicting the corneal penetration of drug molecules.
A principal objective of the present invention is to provide a method for identifying drug molecules having the physical properties required for significant levels of corneal penetration.
A further objective of the present invention is to provide a method for differentiating or ranking drug molecules within a specified class (e.g., fluoroquinolones) based on the abilities of the individual molecules to penetrate the cornea.